It is well known that thunderstorms present a serious threat to airborne craft as a result of the dangerous turbulence, up and down drafts, wind shear and other violent atmospheric disturbances generated thereby. Thunderstorms for the most part are comprised of clusters of cells which grow and dissipate within the storm through a variety of stages. Most of the violent lightning occurs coincidentally with the violent atmospheric disturbances during the mature stage of the storm cells. This strong correlation between the threatening atmospheric disturbances and electrical discharges was used by Ryan to create a weather mapping system known as Stormscope.RTM., which was disclosed in the U.S. Pat. No. 4,023,408 and which is currently assigned to the same assignees as the instant application.
The Ryan system is capable of measuring pulse type electromagnetic radiation generated through the atmosphere from the large electric currents flowing within the lightning channel during a lightning stroke for the detection thereof. Ryan's system used an inverse relationship of the measured radiation to map the detected lightning strike on a display in range and bearing relative to an observation location which, for example, may be the location of the aircraft. In one embodiment, the observation location was calibrated at the center of the display screen and each displayed stroke appeared as a substantial point source at a bearing and radial dimension from the screen center, the radial dimension being proportional to the range measurement of the corresponding lightning stroke. The dimension between the displayed stroke and screen center was not necessarily a measure of the actual range from the aircraft to the stroke but rather an approximation of range based on a mix of distance and intensity information of the detected lightning stroke.
In addition, a U.S. Patent bearing the number U.S. Pat. No. 4,672,305 and issued to Coleman is directed to a lightning detection system which uses a ratio of low (1.5 kHz) and high (500 kHz) frequency magnetic field components to extend the range thereof. Further, U.S. Pat. No. 4,803,421, and its divisional counterpart U.S. Pat. No. 4,873,483, both issued to Ostrander and assigned to the same assignee as the instant application, are directed to lightning detection and mapping systems which determine lightning locations from the ratio of the integrated intensity of two different field components of lightning generated signals. Also, a data acquisition system for use in gathering lightning strike data is present in the paper "A Lightning Data Acquisition System", authorized by B. M. Stevens, Jr. et al. for the International Aerospace and Ground Conference on Lightning and Static Electricity at Dayton, Ohio, Jun. 24-26, 1986.
Another U.S. Patent bearing the number U.S. Pat. No. 5,295,071 and currently assigned to the same assignee as the present application, uses a filter-based method for estimating the location of the lightning strike. The lightning strike signal is passed through a series of narrow-band filters and the filter outputs are used to determine the range and bearing of the lightning strike.
Although each of the systems discussed above offers significant advantages in the detection and mapping of lightning strikes, there remains areas for improvement. For example, many of these systems estimate the location of each lightning strike based on single-strike processing algorithms, i.e., processing which determines the location of a strike independent of the location estimates for other detected strikes. An area for improvement is a system which clusters the individual strikes into thunderstorm cells so that the overall cells may be ranged. Specifically, an area for improvement is a system in which the location of thunderstorm cells may be ascertained based on a history of lightning strikes. The location of the respective thunderstorm cells can then be displayed representing clusters of individual lightning strikes. Such a display provides a clear indication of the location of the thunderstorm cells.